This invention relates to a system for automatically positioning a plurality of tool-holding carriages relative to each other along an elongate guide member. More particularly, the invention relates to, but is not limited to, the positioning of carriages of the type which hold shearing, scoring or creasing tools in an array transverse to a longitudinally-moving web of paper, or which hold any other tools or instruments requiring variable placement with respect to a material or object.
One type of automatic system used in the past for positioning an array of tool-holding carriages employs an individual positioning motor on each carriage, as exemplified by the systems shown in U.S. Pat. Nos. 3,176,566 and 4,649,782, and British patent No. 2,072,563. The requirement for a separate positioning motor on each carriage, however, adds significantly to the expense, complexity of control, and maintenance requirements of the system, and limits the amount of power available to move each carriage.
Another previous type of automatic positioning system employs a single motorized drive for positioning a plurality of tool-holding carriages with respect to each other. Exemplary of this category of systems are those shown in U.S. Pat. Nos. 3,540,340, 3,834,258, 3,882,764, 4,125,076, 4,188,846, 4,261,239 and 4,592,259. These utilize drive members, consisting of elongate screws, sliding bars and the like, capable of moving all of the carriages simultaneously and positioning each carriage by selective engagement and disengagement of the carriage relative to the drive member. However, all of these systems have significant deficiencies with respect to their speed and accuracy of positioning, adversely affecting their productivity and reliability. In most of these systems, the limitation on speed is a result of the inability of the drive member to move the respective carriages bidirectionally simultaneously when repositioning them. Rather, all carriages must usually be moved first in one direction to position some of them, and thereafter in the opposite direction to position the remainder, requiring a time-consuming multi-step process. Even a system such as that shown in U.S. Pat. No. 4,125,076, which utilizes upper and lower reciprocating sliding bars moving in opposite directions to move the carriages bidirectionally simultaneously, cannot provide an improvement in speed because the positioning motion is intermittent rather than continuous due to the reciprocating action of the bars. Moreover, the multiple clutch engagements and disengagements necessary to move a carriage in a single direction by means of a reciprocating drive member can cause significant accumulated positioning errors, as well as excessive wear on the system.
Another limiting factor of all prior automatic systems, affecting both speed and accuracy, is the employment of pulse-counting position sensors for indicating the position of each carriage. Such position sensors, as shown for example in U.S. Pat. No. 3,540,340, indicate carriage position only relative to a reference or "home" position by counting pulses as the carriage moves relative to the reference position. Such pulses can be skipped or missed by a counter, thereby adversely affecting accuracy. Moreover, if power is interrupted for any reason, the stored pulse counts of all of the position sensors are lost, requiring time-consuming return of all of the carriages to the reference position for recalibration of the position sensors before positioning can be determined or repositioning can occur.
A further problem of prior automatic systems adversely affecting their accuracy involves the design of their brake structures, by which each carriage is locked to the guide member to maintain it in proper position. Such brakes normally comprise carriage-mounted brake shoes or pads, or comparable frictional engagement members, which extend slidably into contact with a surface of the guide member in a direction transverse to the positioning direction when the brake is actuated. The slidable interconnection of the engagement member with its carriage, which permits the transverse extending motion, creates a loose tolerance between the engagement member and carriage in the direction of positioning. This in turn permits a degree of latitude in carriage position relative to the engagement member, permitting the carriage to assume slightly different positions within a limited range, rather than a single precise position, when the brake is engaged.
Slitting knives and similar implements for cutting moving web material such as paper produce a substantial amount of particulate matter in the air surrounding the carriages and the guide member. In most prior automatic positioning systems, the mating surfaces interconnecting the carriages with the guide member and drive member, respectively, are exposed to these particles which allows them to clog and abrade the precisely-machined mating surfaces, impeding movement and causing wear. On occasion, foldable bellows-type covers have been used between the carriages in an attempt to protect these surfaces from exposure to particles, but such covers prevent the positioning of the respective carriages in close proximity to each other, and also require substantial maintenance to keep them in good repair.
All of the previous automatic systems have electrical and pneumatic conduits leading to the respective carriages which, because of the movability of the carriages, present a substantial problem of conduit control to avoid excessive wear, kinking, and exposure of the conduits to external hazards. Moreover, the space required to accommodate the guide member, its respective carriages and multiple moving conduits is usually excessively large, making installation difficult and interfering with the space requirements of the associated web-handling equipment.
Where upper and lower single-drive automatic positioning systems are employed on the top and bottom sides of the web material, the space problem is aggravated further by the fact that the upper and lower drive systems are mechanically interconnected so as to be driven in synchronization by a single motor, as exemplified by the systems shown in U.S. Pat. Nos. 3,540,340, 3,882,764, 4,188,846, 4,261,239, and 4,592,259. Furthermore, such interconnection of the two drive members makes maintenance and replacement of the respective upper and lower drive members, and variation of their positional relationship to each other, more difficult since they are not mechanically independent.
Finally, the manner in which previous automatic systems precisely position the multiple carriages in an automatic fashion has required both wasted motion and highly complex computer software to ensure that the respective carriages do not obstruct each other's movements during the positioning process, detracting from speed as well as reliability.